The ancient fire temple is located on the top of a unique mound to the West of Esfahan. The overall structure is 107 metres long and resembles an impregnable castle. Archaeological research in 1967 A.D revealed two strong mud walls that had survived from the 6th century B.C. The fire temple, in which an eternal flame was lit, dates back to the 3rd century B.C. At the top there is a circular fire altar that  5  metres in diameter surrounded by a wall cut with eight windows. It is thought that this structure had a tall dome to protect the fire over long periods.